# std::chrono::round(std::chrono::duration)

From cppreference.com
< cpp‎ | chrono‎ | duration

Defined in header `<chrono>` |  |
template< class ToDuration, class Rep, class Period > constexpr ToDuration round( const std::chrono::duration<Rep, Period>& d ); |  | (since C++17)

Returns the value `t` representable in `ToDuration` that is the closest to d. If there are two such values, returns the even value (that is, the value `t` such that t % 2 == 0).

The function does not participate in the overload resolution unless `ToDuration` is a specialization of std::chrono::duration and std::chrono::treat_as_floating_point_v<typename ToDuration::rep> is false.

### Parameters

d | - | duration to convert

### Return value

d rounded to the nearest duration of type `ToDuration`, rounding to even in halfway cases.

### Possible implementation

    namespace detail
    {
        template<class> inline constexpr bool is_duration_v = false;
        template<class Rep, class Period> inline constexpr bool is_duration_v<
            std::chrono::duration<Rep, Period>> = true;
    }

    template<class To, class Rep, class Period,
             class = std::enable_if_t<detail::is_duration_v<To> &&
                    !std::chrono::treat_as_floating_point_v<typename To::rep>>>
    constexpr To round(const std::chrono::duration<Rep, Period>& d)
    {
        To t0 = std::chrono::floor<To>(d);
        To t1 = t0 + To{1};
        auto diff0 = d - t0;
        auto diff1 = t1 - d;
        if (diff0 == diff1)
        {
            if (t0.count() & 1)
                return t1;
            return t0;
        }
        else if (diff0 < diff1)
            return t0;
        return t1;
    }

### Example

Run this code

    #include <chrono>
    #include <iomanip>
    #include <iostream>

    int main()
    {
        using namespace std::chrono_literals;
        std::cout << "Duration\tFloor\tRound\tCeil\n";
        for (using Sec = std::chrono::seconds;
            auto const d : {+4999ms, +5000ms, +5001ms, +5499ms, +5500ms, +5999ms,
                            -4999ms, -5000ms, -5001ms, -5499ms, -5500ms, -5999ms})
            std::cout << std::showpos << d << "\t\t"
                      << std::chrono::floor<Sec>(d) << '\t'
                      << std::chrono::round<Sec>(d) << '\t'
                      << std::chrono::ceil <Sec>(d) << '\n';
    }

Output:

    Duration	Floor	Round	Ceil
    +4999ms		+4s	+5s	+5s
    +5000ms		+5s	+5s	+5s
    +5001ms		+5s	+5s	+6s
    +5499ms		+5s	+5s	+6s
    +5500ms		+5s	+6s	+6s
    +5999ms		+5s	+6s	+6s
    -4999ms		-5s	-5s	-4s
    -5000ms		-5s	-5s	-5s
    -5001ms		-6s	-5s	-5s
    -5499ms		-6s	-5s	-5s
    -5500ms		-6s	-6s	-5s
    -5999ms		-6s	-6s	-5s

### See also

duration_cast (C++11) | converts a duration to another, with a different tick interval (function template)
floor(std::chrono::duration) (C++17) | converts a duration to another, rounding down (function template)
ceil(std::chrono::duration) (C++17) | converts a duration to another, rounding up (function template)
round(std::chrono::time_point) (C++17) | converts a time_point to another, rounding to nearest, ties to even (function template)
roundroundfroundllroundlroundflroundlllroundllroundfllroundl (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) | nearest integer, rounding away from zero in halfway cases (function)

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